Wellbore Gravity Water Separator

ABSTRACT

A method and apparatus to mitigate water loading in wellbores to increase oil and gas production and/or extend the life of the well. Beginning with a production wellbore through a subterranean formation used to extract hydrocarbons from a reservoir, a water draw-off wellbore, through the subterranean formation on a low-side of the production wellbore, is created to direct water to a non-producing formation. Fluids are produced through the production wellbore at a rate sufficient to lift the fluids to at least an entrance of the draw-off wellbore and allow the water to separate from the hydrocarbons due to gravity. The separated water is directed through the draw-off wellbore to the non-producing formation. Configurations of dual and multi-lateral production and draw-off wellbore arrangements are presented.

The present invention relates to removing water from wellbores in oil and gas production and in particular, though not exclusively, to a method and apparatus for separating and disposing of water in a wellbore to mitigate water loading.

Underground pools of natural hydrocarbons, known as ‘reservoirs’, contain oil and/or gas. These hydrocarbons accumulate within the subterranean rock formations and are recovered or produced therefrom through wells, called production wells, drilled into the subterranean formation.

Recovery occurs by inducing a pressure drop at the production wells which causes the hydrocarbons to expand up the well to the surface. Production will cease when the head of pressure of the fluids in the well equals the pressure in the reservoir, i.e. there is no longer sufficient energy within the reservoir to drive out the hydrocarbons. Such recovery is often termed ‘primary’. The point when the reservoir runs out of energy can be delayed via the natural influx of water from an underlying aquifer, a source of energy, or by the direct injection of water through injection wells, or both. The water not only augments the reservoir pressure it also displaces, or ‘sweeps’ the hydrocarbon out of the rock pore spaces. Such supplemental techniques to obtain additional hydrocarbons are often referred to as secondary, tertiary, enhanced or post-primary recovery operations.

While water can play an important role in recovering hydrocarbons, it's presence in the wellbore can also reduce the rate of production. Indeed, production will cease when the head of the liquids hydrostatic column pressure equals the reservoir pressure. This is referred to as water loading.

In a gas well, for example, water can be entrained as a condensation and precipitate in the wellbore due to any drop in pressure. This is in addition to the water from an aquifer or injection water. This water is not a problem as long as the there is sufficient energy in the well to lift the water through the wellbore to the surface.

If the energy of the well is not sufficient to lift all of the water then the water will start to accumulate at the base of the well. Eventually there will be sufficient water to ‘kill’ the well as there is not enough energy to lift any of the fluids and the well stops production.

In order to maintain production, a method of artificial lift must be employed to bring the fluids from the wellbore to surface treating equipment. At surface the hydrocarbons can then be separated from the water. There are a number of techniques available for artificial lift such as pumps, gas lift, velocity string, foaming, injection system, swabbing and plunger.

There are a number of disadvantages in using these techniques. Many require a rig workover where production must be stopped while additional equipment is installed in the well. This is both time consuming and expensive. Others require that the well is shut-in for a period of time to allow water to run back into the reservoir and allow the local reservoir pressure to build up again. This is again costly as, without production there is no revenue generated from the well. Some will inject foaming agents into the well, which are expensive and can be hazardous to handle.

All the techniques also have the single disadvantage in that the water must be lifted to the surface, separated from the hydrocarbons and then disposed of. Some water can be re-injected into the well, but the remaining water must be disposed of. Due to stringent controls on the purity of produced water which can be dumped in the sea, the separation of water and it's disposal on offshore or onshore production facilities is an ongoing problem in the industry.

It is an object of the present invention to provide a method and apparatus for separating and disposing of a proportion of water in a wellbore which does not artificially lift the water back to the surface for separation and disposal.

It is a further object of at least one embodiment of the present invention to provide a method and apparatus for separating and disposing of a proportion of water in a wellbore which does not require the well to be shut-in.

It is a still further object of at least one embodiment of the present invention to provide a method and apparatus for separating and disposing of a proportion of water in a wellbore which does not require the injection of any chemicals or agents while not precluding their use, if desired.

According to a first aspect of the present invention there is provided a method for separating and disposing of a proportion of water in a wellbore, the method comprising the steps:

-   -   (a) providing at least one production wellbore through a         subterranean formation to extract hydrocarbons from a reservoir;     -   (b) providing at least one water draw-off wellbore through the         subterranean formation on a low-side of the production wellbore         to direct water to a non-producing formation;     -   (c) producing fluids through the production wellbore at a rate         sufficient to lift the fluids to at least an entrance of the         draw-off wellbore and allow the water to separate from the         hydrocarbons due to gravity; and     -   (d) directing the separated water through the draw-off wellbore         to the non-producing formation.

Produced fluids (gas and liquids (gas and oil, gas and water, oil and water, oil-gas-water) will separate under gravity in the production wellbore(s). As water is heavier than both oil and gas it will naturally drop out on the low-side of the wellbore. Thus by providing a route for the separated water to be drawn from the produced fluids, the water is separated and disposed of downhole via the draw-off wellbore.

By removing water from the production fluids downhole, the water loading is reduced and consequently, the fluids can be produced for a longer period of time, increasing the life of the well.

Preferably, the method includes the step of further producing the fluids through the production wellbore at a rate sufficient to lift the fluids to at least an entrance of a second draw-off wellbore and allow more water to separate from hydrocarbons due to gravity. In this arrangement, the second draw-off wellbore can collect additional separated water. Advantageously, this step can be repeated for further draw-off wellbores, providing a cascade of draw-off wellbores, each drawing off additional amounts of separated water. Preferably, the method includes the step of directing the separated water through the draw-off wellbore to a deeper part of the reservoir. Pressure in the draw-off wellbore will be lower than that of the production wellbore as the draw-off wellbore fills with water.

Preferably, the method includes the step of directing the separated water through the draw-off wellbore to an aquifer. In this way the water is returned to a water bearing part of the reservoir.

Advantageously, the draw-off wellbore is formed from a production wellbore. In this way, in a dual or multi-lateral wellbore system, the draw-off wellbore could be, initially, a production wellbore in which the pressure of that lateral has fallen to a point where it preferentially allows water to fall back down its length. Thus draw-off wellbores can act as production wellbores until such time as water is accumulating in them.

Optionally, the method steps (a) and (b) may be reversed, when a production wellbore preferentially takes on water. The method steps will provide for the drilling a new production wellbore such that the previous production wellbore lies in the low side and now acts as the draw-off wellbore. In this way, remedial work can be done to a mature well, to extend it's life. This is in contrast to the current remedial techniques which do not consider the location of the previous production wellbore when a new production wellbore is drilled. Indeed, in some cases the previous production wellbore is plugged before the new production wellbore is produced.

According to a second aspect of the present invention there is provided apparatus for separating and disposing of a proportion of water in a wellbore comprising: at least one production wellbore through a subterranean formation to extract hydrocarbons from a reservoir; at least one water draw-off wellbore through the subterranean formation; wherein the draw-off wellbore is located on the low-side of the production wellbore to direct separated water from produced fluids to a non-producing formation.

It is well known that water, being heavier that oil or gas, will gravitate to the lower side of a deviated well. Therefore fitting (drilling) a draw-off wellbore on the low side of the wellbore will simply allow water to fall down the draw-off wellbore and not accumulate at the base of the production well.

Preferably, the production wellbore is a deviated wellbore. This provides easy accumulation of the water. Alternatively, the production wellbore and the draw-off wellbore are arranged as a dual vertical well with the draw-off perfectly vertical and the production wellbore off at the side and then vertical.

In this way, the vertically arranged draw-off would accumulate the water and not produce allowing the production arm to drop its water into the vertical draw-off wellbore.

Preferably, the draw-off wellbore is completed in a water leg. Alternatively, the draw-off wellbore is completed in the same production reservoir zone as the production wellbore.

Optionally, the draw-off wellbore is completed in an alternative lower pressure formation.

Preferably, there is a plurality of production wellbores providing a multi-lateral wellbore system, the majority of well bores drilled on a low-side of a lateral. In this way, as water production starts in any one or more of the laterals then water has the potential to segregate out and drop to the low side of the lateral and then drop down the low side draw-off lateral. This low pressure lateral thus becomes a draw-off wellbore.

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings of which:

FIGS. 1( a)-(c) are schematic illustrations of a production wellbore and a water draw-off wellbore according to embodiments of the present invention; and

FIG. 2 is a schematic illustration of a multi-lateral wellbore system according to an embodiment of the present invention; and

FIG. 3 is a schematic illustration of an alternative multi-lateral wellbore system according to an embodiment of the present invention.

Reference is initially made to FIG. 1( a) of the drawings which illustrates a wellbore system, generally indicated by reference numeral 10, according to an embodiment of the present invention. System 10 comprises a production wellbore 12 drilled as a wellbore from the surface (not shown) down into a hydrocarbon bearing subterranean formation 16 of a reservoir 18. The drilling and completion of production wellbore 12 is as known in the art. Wellbore 12 deviates from a deeper wellbore 14 at a branch 20.

Wellbore 14 is drilled and completed as is known in the art. While the wellbore 14 is illustrated as a vertical wellbore, it will be appreciated that it does not need to be so, requiring only to be less deviated than wellbore 12 at the low side 24 of the branch point 20. Wellbore 14 is completed in the deeper part of the reservoir 18. Wellbore 14 is a water draw-off wellbore and is ideally completed in the water leg 22, if it exists, but could be completed in the same production reservoir zone 18 or in an alternative lower pressure formation. Water draw-off wellbore 14 is located on the low side 24 of the production wellbore 12.

FIGS. 1( b) and 1(c) show alternative wellbore system configurations, generally indicated by numerals 10 a and 10 b respectively. Like parts to those of FIG. 1( a) have been given the same reference numerals, but suffixed with ‘a’ or ‘b’, as appropriate. FIG. 1( b) shows a deviated production wellbore 12 a with a draw-off wellbore 14 a at the low-side 24 a. FIG. 1( c) shows a production wellbore 12 c, illustrated with exaggerated dips, and a draw-off wellbore 14 c at the dip 11 being the base of the vertical section 13 of the wellbore 12 b to surface. While water will accumulate in the dips, it will be drawn off by wellbore 14 c.

In use, with the wellbores 12,14 drilled and completed, production will commence from wellbore 12 and possibly wellbore 14, if this was completed in a production reservoir zone. Hydrocarbons, typically in the form of gas exist in the reservoir 18 at a certain pressure related to the depth of the formation 16. A pressure drop in the wellbore 14 is created by opening the well at surface. Consequently, the gas 26 enters the wellbores 12,14 and rises to the surface.

During production, water can be entrained as a condensation and precipitate in the wellbore 12 due to a drop in pressure from the formation 16. Additionally water can enter the gas stream if water is present in the formation 16. If the energy of the gas stream in the wellbore 12 is not sufficient to lift all of the produced fluids 30, as water is heavier than hydrocarbons, the water will separate from the production fluid 30 and accumulate in the base of the wellbore 12.

In the present invention, the flow rate of the gas 26 is calculated to be able to lift the produced fluids 30 to at least the branch 20. The water will then separate from the gas 26 and fall down the draw-off wellbore 14, due to gravity and the deeper positioning of the wellbore 14. The water in the draw-off wellbore 14 will act as a manometer and support the column of water equal to the pressure head of the deeper reservoir. The pressure at the top of the water column will then be reduced to equal to that of the production wellbore 12, so that the gas is produced in favour of the water. With the water 28 removed from the fluids 30, the gas 26 will have sufficient energy to be produced at the surface. The water 28 falling into draw-off wellbore 14 will re-enter the formation 22.

Thus a majority of the water is not taken to the surface for separation but disposed of downhole.

It is noted that the use of the draw-off wellbore 14 is in direct contrast to the typical deviated well arrangement currently in use. Current methodology teaches that as production drops, the wellbore 14 will typically be plugged at a location below the branch 20. This is believed to increase production from the wellbore 12. However such action will suffer the same water loading to the wellbore 12 as water may separate under gravity and drop back down the production wellbore 12. This may ultimately ‘kill’ the well unnecessarily. Thus the use of the draw-off wellbore 14, provides longevity to the well.

FIG. 1 also illustrates an alternative embodiment of the present invention. In this embodiment, wellbore 14 is first drilled as a production wellbore. Hydrocarbons will be produced from the wellbore 14 until such time as water loading affects production. Reservoir engineers will then identify a fresh hydrocarbon bearing formation and a new production wellbore 12 will be drilled to access this. The trajectory of the new wellbore 12 will be designed to provide a low-side at the location of the wellbore 14. With wellbore 14 now containing water, when production begins from wellbore 12, wellbore 14 will become the draw-off wellbore. This remedial action offers increased longevity to mature wells.

Reference is now made to FIG. 2 of the drawings which illustrates a section of a multi-lateral well system, generally indicated by reference numeral 110, according to a further embodiment of the present invention. Like parts to those of FIG. 1 have been given the same reference numeral with the addition of ‘100’ to aid clarity. The drilling and completion of the multi-lateral well system 110 is as known in the art.

The wellbores 112,114 are in forked, stacked and/or dual opposing configurations, with the production wellbores 112 being drilled and completed in calculated hydrocarbon bearing formations 116. Each of the draw-off wellbores 114 is located on the low side of a production wellbore 112 and completed in a deeper part of the hydrocarbon reservoir 118 than the production wellbore 112 from which it depends.

Initially, some of the draw-off wellbores 114 may produce hydrocarbons, until such time as the water accumulation in these wellbores 114 is sufficient to cause water loading. When this occurs, the water will fall back under gravity, down the wellbore 114, effectively killing production in the wellbore 114.

It should be noted that where a wellbore is drilled to only act as a draw-off wellbore 114, the location is selected to provide good run off capability to a non-producing zone of the reservoir 118 and not to provide any significant production. This is in contrast to a wellbore drilled into a producing zone which might or will become non-productive as water accumulates in the wellbore. Either or both scenarios can be present in the drilling program.

Produced fluids are drawn up the production wellbores 112 to surface 32. As the well 110 matures, water will be produced with the fluids. Gravity acts on the produced fluids and, as water is heavier than the hydrocarbons, the water will tend to ‘drop-out’ of the produced fluids and gravitate to the low-side of the wellbore 112. Each of the draw-off wellbores 114 is designed to be located on the low-side of the production wellbore 112 from which it depends with the branch point 120 at a position where the water will have been lifted that far while still having sufficient time for the water to separate from the produced fluids. The separated water will thus fall back into the draw-off wellbore 114 and be diverted back into the reservoir 118.

Reference is now made to FIG. 3 of the drawings which illustrates an alternative multi-lateral well system, generally indicated by reference numeral 210, according to a further embodiment of the present invention. In this embodiment, the production well 212 is provided with a number of draw-off wellbores 214, cascading along it's lower-side 224. Using multiple draw-off wellbores 214 increases the proportion of water that is drawn off. The multiplicity of wellbores 214 can be drilled at the same time, or may be drilled as the well matures and water loading prevents the gas being sufficient to lift the fluids to the last/uppermost draw-off wellbore 214 a.

It is appreciated that the position of the branch points 120 is difficult to precisely determine, but the mere presence of a draw-off wellbore 114 is beneficial to water removal.

For the laterals at higher locations in the well 110, water can also run off into the main arterial draw-off wellbore 114 a, due to it's substantially vertical orientation.

The principle advantage of the present invention is that it provides a method and apparatus for separating and disposing of a proportion of water in a wellbore which does not lift the water back to the surface and can extend the production life of a well.

A further advantage of at least one embodiment of the present invention is that it provides a method and apparatus for separating and disposing of a proportion of water in a wellbore which does not require the well to be shut-in.

A yet further advantage of at least one embodiment of the present invention is that it provides apparatus for separating and disposing of a proportion of water in a wellbore which can use existing wellbores or can be retrofitted when water loading becomes a problem.

A still further advantage of at least one embodiment of the present invention is that it provides a method and apparatus for separating and disposing of a proportion of water in a wellbore which does not require the injection of any chemicals or agents while not precluding their use, if desired.

It will be appreciated by those skilled in the art that modifications may be made to the invention herein described without departing from the scope thereof. For example, other multi-lateral configurations can be used including any number of laterals, providing at least one draw-off wellbore is located on a low-side of at least one of the production wellbores. While a gas well has been described any hydrocarbon producing well may be used. Other existing methods for producing water loaded wells might also be used alongside the present invention. Additionally, chemicals or agents can be used to assist in the separation of the water, for example, you could inject demulsifier to help separate the water in the wellbore, especially in oil wells. 

1. A method for separating and disposing of a proportion of water in a wellbore, the method comprising the steps: (a) providing at least one production wellbore through a subterranean formation to extract hydrocarbons from a reservoir; (b) providing at least one water draw-off wellbore through the subterranean formation on a low-side of the production wellbore to direct water to a non-producing formation; (c) producing fluids through the production wellbore at a rate sufficient to lift the fluids to at least an entrance of the draw-off wellbore and allow the water to separate from the hydrocarbons due to gravity; and (d) directing the separated water through the draw-off wellbore to the non-producing Formation.
 2. A method according to claim 1, wherein the method includes the step of further producing the fluids through the production wellbore at a rate sufficient to lift the fluids to at least an entrance of a second draw-off wellbore and allow more water to separate from hydrocarbons due to gravity.
 3. A method according to claim 2, wherein this step is repeated for further draw-off wellbores, providing a cascade of draw-off wellbores, each drawing off additional amounts of separated water.
 4. A method according to claim 1, wherein the method includes the step of directing the separated water through the draw-off wellbore to a deeper part of the reservoir.
 5. A method according to claim 1, wherein the method includes the step of directing the separated water through the draw-off wellbore to an aquifer.
 6. A method according to claim 1, wherein the draw-off wellbore is formed from a production wellbore.
 7. A method according to claim
 6. wherein the method steps (a) and (b) are reversed as the production wellbore preferentially takes on water.
 8. Apparatus for separating and disposing of a proportion of water in a wellbore comprising: at least one production wellbore through a subterranean formation to extract hydrocarbons from a reservoir; at least one water draw-off wellbore through the subterranean formation; wherein the draw-off wellbore is located on the low-side of the production wellbore to direct separated water from produced fluids to a non-producing formation.
 9. Apparatus according to claim 8, wherein the production wellbore is a deviated wellbore.
 10. Apparatus according to claim 8, wherein the production wellbore and the draw-off wellbore are arranged as a dual vertical well with the draw-off perfectly vertical and the production wellbore off at the side and then vertical.
 11. Apparatus according to claim 8, wherein the draw-off wellbore is completed in a water leg.
 12. Apparatus according to claim 8, wherein the draw-off wellbore is completed in the same production reservoir zone as the production wellbore
 13. Apparatus according to claim 8, wherein the draw-off wellbore is completed in an alternative lower pressure formation.
 14. Apparatus according to claim 8, wherein there is a plurality of production wellbores providing a multi-lateral wellbore system, the majority of well bores drilled on a low-side of a lateral with at least the lost pressure lateral being a draw-off wellbore. 